The capacity of WDM systems has been growing at a rate surpassing Moore's Law. Nevertheless, while telecommunication networks are evolving towards packet-switching, WDM systems still remain largely circuit-switched. Fast wavelength tunability is both a challenge and key to true packet-switched WDM networks. A tunable laser opens the possibility to connect from any WDM node to any other WDM node with a single transmitter, thus enhancing network flexibility and enabling smooth network upgrades. While tunable Distributed Bragg Reflector (DBR) lasers with nano-second wavelength tuning speed and fast wavelength insensitive optical switches with gigahertz responses are becoming commercially available, there seems to be no current cost-effective way to reconfigure wavelength add-drop multiplexers at an adequate speed. DBRs are a special type of laser mirror, which reflect light only in a narrow frequency band and allow tunable laser operation.
The previous work on photonic slot routing, as described in “Scalable WDM access network based on Photonic Slot Routing” by I. Chlamtac, V. Elek and C. Szabó published in IEEE Transactions on Networking, Vol. 7, No. 1, 1999, pp 1–9, implies distributed generation of packets, which implies more complicated scheduling of the packets. FIG. 1 is a simplified diagram of Chlamtac's proposal. The upper ring, with packets propagating counter-clockwise, is the core optical ring 110. The lower ring, with packets propagating clockwise and which Chlamtac refers to as the segment ring, will be herein referred to as subtending ring 120. The small rectangular boxes are nodes on each ring. Nodes on the core optical ring are denoted as 105. A 2×2 switch 125 is between the core optical ring 110 and a subtending ring 120. There is a plurality of nodes on a subtending ring, and at each node 115 on the subtending ring, there is a receiver tuned to a fixed optical wavelength. Multiple tunable lasers generate packets, with each tunable laser contributing one packet at one wavelength. That is, the generation of new packets is distributed, with each node capable of generating new packets. The system proposed by Chlamtac is lossy because Chlamtac uses power splitters since there are no Optical Add/Drop Multiplexers (OADMs), capable of operating at an adequate speed and power splitters are intrinsically lossy. That is, there is admission loss when the power splitters add channels. There is a significant under-utilization of the tunable lasers in the system proposed by Chlamtac because the lasers are used to generate only one packet at one wavelength.
There is a further necessity for the core optical ring in Chlamtac's proposed system to synchronize with a subtending ring, which is problematical in light of the optical buffers required by a subtending ring. The problem is exacerbated because there can be multiple subtending rings in the network, each of which require optical buffering. In Chlamtac's proposed system, an entire composite packet is dropped at a node on a subtending ring and the first node removes a packet (a portion of the composite packet) in which the node has an interest. The node can then add a new packet at the same wavelength or any other vacant wavelength, where a vacant wavelength is a wavelength not already present in the composite packet. If the first node of a subtending ring removes a packet of the composite packet and adds another packet to the packet, then a subsequent node on the subtending ring can read or inspect the added packet, which means that privacy is lost. This privacy loss may be unacceptable for certain applications.
The proposal described by Chlamtac also does not provide a way of dropping a part of an optical composite packet from the core ring. That is, the entire composite packet must travel around the subtending ring. Transparent bypass is a scheme where each node is transparent to those wavelengths in a packet that do not match the wavelengths present in the set of fiber Bragg gratings (FBGs) at the node. The concept of transparent bypass (and concomitant bandwidth re-utilization) was not disclosed by Chlamtac. Separate lasers are needed to generate different wavelengths, which means under-utilization of resources, and no implementation of the stacking means described herein was proposed. Distributed generation of packets implies a hierarchical synchronization scheme. Finally, the ability to tune the add/drop filters was not envisioned.